Systems for storing a fluid in liquefied form and delivering the fluid in gaseous form are gaining favor for delivering fuel to internal combustion engines. Internal combustion engines have been used to produce power and drive machines for over a century. Historically, gasoline and diesel have been preferred fuel choices because they were abundant, inexpensive, and easy to store. While natural gas has been used as a fuel for vehicles for over fifty years, widespread use has been curtailed for various reasons including fuel storage density, infrastructure, availability of fuel, and capital costs that are generally higher compared to conventional liquid-fuelled vehicles. However, alternative fuel choices are receiving renewed attention because of several factors, including changing economic conditions, the desire to reduce pollution, and the desire to reduce dependency on diminishing and increasingly expensive oil resources.
There are numerous advantages to substituting liquid fuels with natural gas or other gaseous fuels that are combustible in an internal combustion engine and which are in the gaseous phase at standard pressure and temperature conditions. “Standard pressure and temperature” are defined herein to be an absolute pressure of 1 bar (14.5 psi) and 0 degrees Celsius (32 degrees Fahrenheit). Natural gas is a mixture of combustible gases and its exact composition varies depending on the source, but it is ordinarily primarily methane. Other gaseous fuels include ethane, propane, and other lighter flammable hydrocarbon derivatives as well as hydrogen and mixtures thereof. For example, mixtures of hydrogen and natural gas have been used as a fuel for internal combustion engines, and such mixtures are sometimes referred to as “HCNG”. Compared to conventional liquid fuels, the disclosed gaseous fuels are generally cleaner burning and can be produced from renewable sources. Natural gas is one of the most abundant gaseous fuels available today.
Because the available space for storing a fuel on board a motor vehicle is often limited, a factor that inhibits broader adoption of natural gas as a transportation fuel has been the energy density of gaseous fuels compared to liquid fuels. For example, natural gas stored in gaseous form, at a storage pressure of about 200 bar (3000 psi) requires a volume that is about 4 times the volume required to store the same amount of energy as diesel or gasoline. A solution for increasing the energy density of a gaseous fuel is to store it in liquefied form. For example, if the liquefied natural gas (“LNG”) is stored at low temperatures, for example between about −150° C. and −130° C. (between about −240° F. and −200° F.), this can be achieved with a relatively low storage pressure (for example, pressures between about 1 and 13.8 bar (between about 14.5 and 100 psi)) for LNG. However, for systems that store a fluid such as natural gas in liquefied form and that deliver it in a gaseous form, a solution is required for reducing the venting of pressurized gaseous fluid from the system, under certain conditions, such as when the system is shut down or when the pressure in the fuel supply line is higher than a desired pressure commanded for operating the engine as being determined for example by reference to the engine map. In gas delivery systems that supply gases such as oxygen or nitrogen, while wasteful if high pressure gas needs to be relieved from the system, the pressurized gas can be simply vented to atmosphere without concern for its effect on the environment. In gas delivery systems that supply fuel to an engine, while natural gas is a naturally occurring constituent of the earth's atmosphere it normally exists in very small amounts and it is both undesirable and wasteful to vent natural gas to atmosphere. Accordingly, for fuel delivery systems that supply a gaseous fuel such as natural gas to an engine or other end user, even though the amount of gas present in a supply line is small, it is preferred to reduce the amount of natural gas that is vented from the supply line to the atmosphere.
Liquefied gaseous fuels, such as LNG, are stored in thermally insulated storage vessels to reduce heat leak into the storage space. Heat that leaks into the storage space can cause vaporization of some of the stored fluid, thereby increasing the vapor pressure. If the vapor pressure exceeds the predetermined relief pressure for the storage vessel, a pressure relief valve opens to prevent damage to the storage vessel. Typically the vapor vented from the storage vessel is released into the atmosphere. To minimize fuel venting to the atmosphere, LNG storage vessels include features for preventing heat from being introduced into the storage space and/or features to prevent increasing the vapor pressure in the storage vessel above the relief pressure.
Previous approaches for vehicular fuel systems have taught away from returning a fluid in gaseous form back to a storage vessel where the fluid is stored in liquefied form to thereby avoid increasing the pressure in the storage vessel above the relief pressure. For example, instead of recovering fluid in gaseous form, United States Patent Publication number 2009/0095153 teaches directing the vented gas from a storage vessel to a burner or a catalyst. Such solutions add more components and complexity to the system thereby reducing the overall system efficiency and increasing the system's capital and operational cost.
Other known systems that do recover a fluid in gaseous form such as the system described in United States Patent Publication number 2009/0133674 use a method which includes cooling and re-liquefying the fluid before it is returned to the storage vessel or before it is directed elsewhere. The equipment needed to re-liquefy the fluid is expensive, takes up space, and adds more complexity and weight to the system.
Accordingly, there is a need for a simple and more efficient system and method for reducing the amount of fluid in gaseous form that is vented to atmosphere, especially reducing the amount of fuel in gaseous form that is vented from an internal combustion engine.